1. Field of the Invention
The present invention generally relates to time-of-flight (TOF) mass spectrometers, and in particular to a TOF mass spectrometer utilizing rotating electromagnetic field for identifying the chemical composition of low-pressure gases, the relative abundance of different species, and the particle flow direction and velocity.
2. Description of the Related Art
Analysis of the chemical composition of gases plays a central role in our attempt to understand the origin and the evolution of our solar system. In situ analysis of the gas must be considered complementary to remote sensing: while the first type of measurement identifies and quantifies all chemical compounds at a point in space, the second gives a spatial overview of those elements and compounds that generate a unique optical signature, like an emission or absorption line. The enrichment or depletion of a specific element is important information, when trying to understand the processes that control planetary evolution. Investigations of the Martian atmosphere and the coma of comets both rely on detailed chemical composition information. The composition of the Martian atmosphere was characterized during the descent phase of both Viking landers.
Many questions have been left unanswered, in particular the variation of the chemical composition associated with the dust content of the atmosphere, as well as the coupling between atmospheric composition and climate variations. The chemical composition and relative abundance of elements in the atmosphere of comets and small bodies is a rapidly varying function of the solar illumination, latitudinal and longitudinal position, and height from the body's surface. To characterize the changes and relate them to features on the surface, a mass spectrometer must show at the same time high resolving capability and fast response. State of the art mass-spectrometers may not be entirely suitable for measurements of gases where the composition varies rapidly, or where the composition is not known a-priori, and the full mass spectrum has to be analyzed in a relatively short time. Study of the atmospheres and exospheres of planets and small bodies of the solar system is a central point in the NASA programs of the next decade. Chemical and isotopic characterization of these tenuous gases is extremely important when trying to understand origin and evolution of the solar system itself.
Many different techniques have been proposed and implemented during the past century to quantitatively determine the relative and absolute abundance of chemical components. In the realm of low-pressure gases, where a single particle approach is appropriate, atoms and molecules in either neutral or ionized state are analyzed. Neutral particles are usually ionized before the analysis, typically using electron bombardment.
Among numerous types of TOF spectrometers, it is known to use a TOF mass spectrometer utilizing a rotating RF field. In general, this type of mass spectrometer is known for its increased ability to reliably acquire and analyze mass spectra with high sensitivity, high accuracy and high duty cycle, since ions continuously traverse a dispersion system and are continuously analyzed.
U.S. Pat. No. 6,521,887 discloses a TOF mass spectrometer provided with an electrostatic deflection apparatus, which includes multiple dispersing electrodes arranged in consecutive pairs that are spaced angularly relative to one another at approximately right angle. Ions continuously entering a drift tube are first swept by the electrostatic deflection apparatus so that the ion trajectory is a function of the voltage impressed on the dispersing electrodes during the time the ion passes there through. Placing a detector at the predetermined lateral distance from the symmetry axis of the deflection apparatus allows only ions of interest to impact thereupon. This information, when combined with information on the voltages applied and the hit time of the ion detection provides a method for determining the time-of-flight of the detected ion along the drift region having known length. Having determined the time and knowing the distance traveled by the detected ion, one can determine its speed and further its mass-per-charge ratio, assuming that the ion beam has a well known energy-per-charge ratio.
Universally, all types of mass spectrometer are configured to attain, among others, the following objectives (1) high ionization rate; (2) high mass resolution; and (3) compactness.
Additionally, in the context of the TOF mass spectrometer with the rotating electromagnetic field, it is highly important to minimize fringing fields, which detrimentally affect the desired trajectory of deflected ions. A combination of consecutively positioned pairs of dispersing electrodes, as disclosed in U.S. Pat. No. 6,521,887, contributes to rather a space-inefficient structure as well as the presence of substantial fringing fields.
A need therefore exists for a TOF mass spectrometer having a very high mass resolving power (capable of separating different isotopes at the 1% level), an extremely high sensitivity (to be able to measure very diluted gases), and requiring very limited resources indispensable for space applications.